Electrophotographic printing is a well-known and commonly used method of copying or printing documents. Electrophotographic printing is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner is then deposited onto that latent image, forming a toner image. The toner image is then transferred from the photoreceptor onto a receiving substrate such as a sheet of paper. The transferred toner image is then fused with the substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing generally describes black and white electrophotographic printing machines. Electrophotographic printing can also produce color images by repeating the above process for each color of toner that is used to make the color image.
For example, the photoreceptive surface may be exposed to a light image that represents a first color, say black. The resultant electrostatic latent image can then be developed with black toner particles to produce a black toner layer that is subsequently transferred onto a receiving substrate. The process can then be repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. When the toner layers are placed in superimposed registration the desired composite color toner image is formed and fused on the receiving substrate.
The color printing process described above superimposes the color toner layers directly onto a substrate. Other electrophotographic printing systems use intermediate transfer belts. In such systems successive toner layers are electrostatically transferred in superimposed registration from the photoreceptor onto an intermediate transfer belt. Only after the composite toner image is formed on the intermediate transfer belt is that image transferred and fused onto the substrate. Indeed, some electrophotographic printing systems use multiple intermediate transfer belts, transferring toner to and from the belts as required to fulfill the requirements of the machine's overall architecture.
In operation, an intermediate transfer belt is brought into contact with a toner image-bearing member such as a photoreceptor belt. In the contact zone an electrostatic field generating device such as a corotron, a bias transfer roller, a bias blade, or the like creates electrostatic fields that transfer toner onto the intermediate transfer belt. Subsequently, the intermediate transfer belt is brought into contact with a receiver. A similar electrostatic field generating devices then transfers toner from the intermediate transfer belt to the receiver. Depending on the system, a receiver can be another intermediate transfer member or a substrate onto which the toner will eventually be fixed. In either case the control of the electrostatic fields in and near the transfer zone is a significant factor in toner transfer.
Intermediate transfer belts often take the form of seamed belts fabricated by fastening two ends of a web material together, such as by welding, sewing, wiring, stapling, or gluing. While seamless intermediate transfer belts are possible, they require manufacturing processes that make them much more expensive than similar seamed intermediate transfer belts. This is particularly true when the intermediate transfer belt is long. While seamed intermediate transfer belts are relatively low in cost, the seam introduces a discontinuity that interferes with the electrical, thermal, and mechanical properties of the belt. While it is possible to synchronize a printer's operation with the motion of the intermediate transfer belt such that toner is not electrostatically transferred onto the seam, such synchronization adds to the printer's expense and complexity, resulting in loss of productivity. Additionally, since high speed electrophotographic printers typically produce images on paper sheets that are cut from a paper "web," if the seam is avoided the resulting unused portion of the paper web must be cut-out, producing waste. Furthermore, even with synchronization the mechanical problems related to the discontinuity, such as excessive cleaner wear and mechanical vibrations, still exist. However, because of the numerous difficulties with transferring toner onto and off of a seamed intermediate transfer belt, some of which are discussed below, in the prior art it was necessary to avoid toner transfer onto (and thus off of) the seam.
Acceptable intermediate transfer belts require sufficient seam strength to achieve a desired operating life. While the desired operating life depends on the specific application, typically it will be at least 100,000 operating cycles, and more preferably 1,000,000 cycles. Considering that a seamed intermediate transfer belt suffers mechanical stresses from belt tension, traveling over rollers, moving through transfer nips, and passing through cleaning systems, achieving such a long operating life is not trivial. Thus the conflicting constraints of long life and limited topographical size at the seam places a premium on adhesive strength and good seam construction.
A prior art "puzzle cut" approach to seamed intermediate transfer belts significantly reduces mechanical problems by producing an improved mechanical seam. U.S. Pat. No. 5,514,436, issued May 7, 1996, entitled, "Puzzle Cut Seamed Belt;" U.S. Pat. No. 5,549,193 entitled "Endless Seamed Belt with Low Thickness Differential Between the Seam and the Rest of the Belt;" and U.S. Pat. No. 5,487,707, issued Jan. 30, 1996, entitled "Puzzle Cut Seamed Belt With Bonding Between Adjacent Surface By UV Cured Adhesive" teach the puzzle cut approach. While puzzle cuts reduce mechanical problems there remains other difficulties with transferring toner onto and off of a seam of a seamed intermediate transfer belt.
For transferring toner onto and off of a seam to be acceptable, the final image produced from across the seam must be comparable in quality to images formed across the remainder of the belt. This is a difficult task due to a number of interrelated factors. Some of those factors relate to the fact that the seam should not greatly impact the electrostatic fields used to transfer toner. However, electrostatic transfer fields are themselves dependent on the electrical properties of the intermediate transfer belt. While this dependency is complex and a more detailed discussion of this subject is given subsequently, briefly there are conditions where transfer fields are very sensitive to the resistivity and thickness of the materials used for the various layers of the intermediate transfer belt. Under other conditions the electrostatic transfer fields are relatively insensitive to those factors. Similarly, there are conditions where the electrostatic transfer fields are very sensitive to the dielectric constants of the materials used for the layers of the intermediate transfer belt, and other conditions where the electrostatic transfer fields are insensitive to the dielectric constants. Therefore, to successfully transfer toner onto and off of a seamed intermediate transfer belt the electrical properties across and around the seam should be carefully controlled to produce a proper relationship with the remainder of the belt. Since the electrical properties depend on the interrelated factors of seam geometry, seam construction (such as adhesive beyond the seam), seam topology, seam thickness, the presence of an overcoating, and various other factors those factors should be taken into consideration for a given application.
From above it can be seen that if toner is to be transferred onto and off of a seam that the critical properties at the seam region must be controlled such that the electrostatic transfer fields across the seam are close to those away from the seam. While conditions that achieve this are discussed in more detail later, generally those conditions involve the use of "forgiving resistivity ranges." However, it should be noted that one only needs to provide seam conditions that result in "sufficiently close" electrostatic transfer fields. Sufficiently close depends on the tolerance of a given system to differences in the electrostatic transfer fields. Experience shows that some systems can tolerate more than a 20% difference in the electrostatic transfer fields without a significant difference in the final image. However, high quality color systems usually must have less than a 10% difference to avoid noticeable problems. However, "sufficiently close" is best determined by experimentation.
Even if the electrical properties of a seamed intermediate transfer belt are suitable for producing acceptable images across the seam region, other problems remain. For example, with prior art seamed intermediate transfer belts relatively poor cleaning and transfer around the seam is acceptable. However, if toner is being transferred onto and off of the seam region the seam must be properly cleaned. Thus, the toner release and friction properties across the seam region would have to be comparable to those of the rest of the belt. Furthermore, most prior art seamed intermediate transfer belts have a significant "step" where the belt overlaps to form the seam. That step can be as large as 25 micons. Such a step significantly interferes with transfer and cleaning. Thus if toner is transferred onto and off of the seam, the seam's friction, toner release, and topography are much more constrained than those of other seamed intermediate transfer belts.
From above it can be seen that a seam's topography is very important if one wants to transfer toner onto and off of a seam region without significant degradation of the final image. The seam topography includes not only the seam itself, but also any overflow of the adhesive used in the seam. This overflow can occur on both the toner-bearing side and the back-side of the belt. Adhesive overflow is important because the belt seam strength can depend upon on that overflow. However, excessive overflow increases various mechanical, electrical, and xerographic problems. Furthermore, the adhesive's electrical properties remain important.
When attempting to transfer toner onto and off of a seam the seam's topography introduces spatial disturbances that are conveniently classified as "short-wavelength" disturbances and "long-wavelength" disturbances. While these disturbances both relate to the mean distance between adjacent peak-to-valley spatial defects, short-wavelength disturbances are small, say less than 3 millimeters, while long-wavelength disturbance are large, say greater than 3 millimeters,. While both disturbances must be sufficiently controlled, short-wavelength disturbances usually require more stringent control than long-wavelength disturbances. Short-wavelength disturbances on the toner-bearing side of the belt are usually much more significant than on the back-side.
Short-wavelength disturbances include, for example, bumps, valleys or steps, kinks or distortions, and peak-to-valley roughness. Such defects are results of the seam type, adhesive overspill, seam manufacturing, or grinding or polishing. One problem with short wavelength disturbances is that they introduce small, unwanted air gaps at the transfer nips. Due to belt stiffness some "tenting" occurs due to short wavelength topography, and the extra air gaps caused by the short wavelength topography can then extend quite far beyond the location of the peak to valley distortion. The unwanted air gaps can be reduced by pressure in the transfer nip. Thus a pressured transfer fleld generation device, such as a conformable bias transfer roller, is generally preferred over a pressureless transfer field generation device, such as a corotron.
Small, unwanted air gaps could be reduced by using an intermediate transfer belt having a conformable overcoat. However, a conformable overcoat can introduce other problems, such as friction or poor electrostatic toner release. Also, for very short-wavelength disturbances, such as a large bump at the seam, the pressure needed to eliminate unwanted air gaps is normally impractical even if a conformable overcoat is used.
On the toner-bearing side small, unwanted air gaps can significantly limit electrostatic transfer fields due to Paschen air breakdown. As known in the art, for air gaps between about 5 microns and 100 microns the maximum field, E.sub.c, that can be supported before breakdown in an air gap d.sub.A decreases with an increasing air gap. This is called Paschen air breakdown and it can be approximately expressed as: E.sub.c =[6.2 Volts/m+(312 Volts)/d.sub.A ]. When an applied E-field in an air gap tries to go above E.sub.c, an air breakdown charge transfer occurs that limits the field to near or below E.sub.c. Since air gaps of 5 to 15 microns can already be present near the edges of and within a toner image, extra air gaps will reduce the maximum E-field that can be present during electrostatic toner transfer of the toner. For example, if air gaps in a toner layer are about 15 microns, Paschen air breakdown will limit the applied electrostatic fields to around 27 volts/micron. However, if an unwanted air gap of 10 microns is introduced by the seam the total air gap increases to 25 microns and the transfer E-field will be limited to around 18.7 volts/micron. While a desirable transfer E-field depends on many factors, air gap transfer E-fields are typically above 20 volts/micron and often above 35 volts/micron.
In addition to transfer problems, short-wavelength disturbances can degrade the effectiveness of cleaning systems. Blade cleaning systems tend to work better with very small short-wavelength disturbances. For example, short-wavelength disturbances of about 0.1 microns can result in reduced friction between the blade and the cleaning surface, thereby helping cleaning.
Therefore, when attempting to transfer toner onto and off of a seam the seam's topography should not introduce transfer nip air gaps above around 10 microns. Preferably unwanted air gap should be less than around 5 microns, and more preferably less than around 1 micron.
When attempting to transfer toner onto and off of a seam without seriously impacting the final image, the seam's long-wavelength disturbances also must be sufficiently controlled to produce an acceptable final image. Examples of unwanted long-wavelength disturbances include "belt ripple" or "belt waviness" longer than 3 millimeters. Long-wavelength disturbances usually are less important than shortwave-length disturbances because a relatively low pressure on a belt can flatten long-wavelength disturbances. Thus it is preferable to use a pressured transfer field generation device, such as a nip-forming bias transfer roller. Also, it is beneficial to tension the belt in cleaning zones such that the belt is relatively flat.
While small disturbances can be significant on the toner-bearing side of a belt, larger backside disturbances can usually be tolerated. First, this is because air gaps introduced by back-side disturbances do not usually cause unwanted air gaps on the toner-bearing side of the belt. Therefore back-side induced Paschen air breakdown is not a major issue. Second, since good back-side cleaning is usually not required the topography constraints related to cleaning are typically not an issue. Finally, for a conformable belt, belt conformance can prevent gaps on the back-side of the belt from being a significant problem. In general, back-side topography should not introduce air gap higher than 10 microns, and preferably it should be less than 5 microns.
While seamed intermediate belts without an overcoat are relatively low cost and relatively simple to manufacture, an overcoat on the toner bearing surface can insure that the seam region has the same toner release and friction properties as the rest of the belt. This enables a wider range of adhesives to be used. Therefore, seamed intermediate transfer belts typically include a substrate layer and an overcoat formed from one or more overcoating layers. Those layers have electrical properties that prevent high voltage drops across the belt, that prevent high pre-nip transfer fields via lateral conduction of the belt, that avoid charge buildup, and that prevent high current flow.
While the electrical properties of a seamed intermediate transfer belt should be controlled so as to integrate that belt with other electrophotographic printer subsystems, acceptable belt resistivities should be typically less than 1.times.10.sup.13 ohm-cm volume resistivity and more than 1.times.10.sup.8 ohms/square lateral resistivity. Lateral resistivity is defined as being the volume resistivity in the direction of belt motion divided by the layer's thickness. In some cases the belt resistivity is sensitive to the applied field. In such cases the volume resistivity should be referenced to a corresponding range of applied fields. While the applied field depends on the particular system design, the upper limit volume resistivity is generally measured at a field corresponding to between 10 to 100 volts across the layer thickness, and the lower limit lateral resistivity of interest is generally measured between 500 to 2000 volts/cm.
Seamed intermediate transfer belts can also have constraints on the lower limit of their volume resistivity in the thickness direction. Typically such constraints occur in systems where the intermediate belt contacts or moves so close to a low resistivity surface in a transfer zone that the possibility of high resistive or corona discharge current density flow between the belt and the low resistivity surface exists. One example of such a system is a drum photoreceptor that has scratches or pin holes in an otherwise insulating drum coating. An intermediate transfer belt can momentarily come very close or even touch the highly conductive drum substrate at the scratches or pin holes in the transfer zone. Another example is a system that transfers toner from one intermediate transfer belt to a second, relatively conductive intermediate transfer receiver. In such systems if the intermediate system composite resistance, R.sub.comp, in the transfer nip is too low, problems can occur due to undesirably high local current density flow between the intermediate transfer belt surface and the low resistivity contacting surfaces in the transfer nip. Problems can include local "shorting" between the intermediate transfer belt surface and the receiver that can cause momentary loss of the local applied electrostatic transfer field, and thereby result in degraded toner transfer. The composite resistance, R.sub.comp, in the transfer nip is the sum of all possible "shorting" resistance paths in the transfer nips. The composite resistance path includes, for example, the effective resistance path of the transfer field generating device, the resistance path of the intermediate belt substrate, and the resistance path of the intermediate belt overcoat.
Shorting issues can be solved by insuring that there is a "sufficiently high" composite resistance path within the transfer nips. Whether a composite resistance is "sufficiently high" depends on the system, and especially on the type of power supply used for the field generating system. The shorting issue occurs when the shorting leakage current flow in the intermediate transfer nips is "too high." The shorting leakage current flow is the applied potential difference in the transfer nip divided by the composite resistance. For example, the current will be "too high" when it exceeds the power supply current capability. Typical power supplies used in transfer systems limit the current to less than 2 milliamps, so such shorting currents are "too high" for most systems. Other power supplies used in transfer systems use constant current power supply control. In such systems, the applied transfer fields are related to the portion of the controlled current that is not shorting leakage current. Thus any shorting leakage current tends to significantly reduce the transfer fields. Typically, with a constant current control, the shorting leakage current will be "too high" when the leakage current exceeds about 20% of the nominal constant current control.
The allowed lower resistivity limit of an intermediate transfer belt also depends on other system inputs. For example, the shorting problem caused by photoreceptor defects depends on the size of the defects that are present in the system. So, in systems that maintain very good defect free high dielectric strength drum coating layers, shorting to drum defects can be avoided even with extremely low volume resistivity intermediate transfer belts. Thus the allowed lower limit for the volume resistivity can vary widely. Still, experience suggests guidelines to avoid shorting problems. To avoid problems in systems that have a "small area shorting contact" in the transfer nip, such as in the drum defect example, the volume resistivity of the topmost layer on the intermediate transfer belt should be above 10.sup.7 ohm-cm, with a preference of being above 10.sup.8 ohm-cm. The resistivity values apply for intermediate material layer thickness that is at least around 25 microns thick or larger. If the resistivity of the materials used for the intermediate transfer belt are sensitive to the applied field, the volume resistivity should be measured with an applied potential difference across the transfer belt that is similar to the applied potential difference used in the transfer system. With low resistivity intermediate materials, this is typically around 200 to 1000 volts across the thickness of the intermediate belt material.
It can be appreciated by those skilled in the art of electrostatic transfer that the electrical properties allowed for any particular intermediate transfer belt application can depend on many factors. Thus some systems can achieve acceptable intermediate transfer performance with intermediate transfer belt material layers having a much higher resistivity than 1.times.10.sup.13 ohm-cm and with materials layers having a much lower lateral resistivity than 1.times.10.sup.8 ohms/square. For example, a problem with very high resistivity intermediate materials layers is charge buildup between transfer stations or belt cycling. However, charge buildup problems can be minimized with belt material layers having much higher resistivity than 1.times.10.sup.13 ohm-cm if suitable charge conditioning devices such as corotrons or scorotrons are provided along the circumference of the intermediate transfer belt configuration to reduce and level the unwanted charge buildup. Generally, with very high resistivity intermediate material layers in color systems, charge conditioning devices are necessary but not sufficient. To be fully effective the total dielectric thickness of any very high resistivity belt layers must also be kept low, typically less than 25 microns, and preferably less than 10 microns. Unwanted cost and complexity is introduced by the need for cyclic charge conditioning devices, and therefore intermediate systems most typically prefer suitably lower resistivity intermediate materials.
Similarly, although not preferred, some systems can use intermediate transfer belts that have material layers on the belt that have lateral resistivity less than 1.times.10.sup.8 ohms/square. Such belts are typically not desired because, if any layer of an intermediate transfer belt has a lateral resistivity somewhat less than 1.times.10.sup.8 ohms/square, high electrostatic transfer fields can occur in the pre-nip region of the transfer zones before contact of the belt with the toner. High pre-nip fields can cause toner transfer across large air gaps in the pre-nip region and this can result in undesirable toner disturbance or splatter of the toner beyond the edges of the image. Also, due to lateral conduction of charge away from the contact transfer nip, any increase in the transfer fields in the contact nip automatically increases the fields in the pre-nip region. This can cause pre-nip air breakdown between the toner and intermediate belt prior to the contact nip. Charge exchange due to pre-nip air breakdown limits the applied transfer fields and it tends to reverse the polarity of any untransferred toner in the pre-nip region. This can then limit transfer efficiency and it can cause image defects due to the nonuniform nature of typical pre-nip air breakdown. However, if the toner adhesion in a particular system is low such that the required electrostatic transfer fields in the nip for good transfer are low, pre-nip field problems caused by lateral conduction can be a small issue. Then, some systems can achieve acceptable transfer performance in spite of having low intermediate belt lateral resistivity.
A complication in enabling transfer of toner onto and off of a seamed intermediate transfer belt is that the electrical properties of an intermediate transfer belt and the seam are generally not constant. For example, the resistivity of most materials used for seamed intermediate transfer belts depend on the fields within the material. Those electrical properties can also depend on the environment, aging, and use. In addition, many manufacturing processes can produce a relatively wide distribution of resistivity values for film materials due to small variations in the resistivity control factors in the manufacturing process. Thus, the materials used for intermediate transfer belts and for the seam adhesives can have resistivities that vary by more than a factor of 100. Therefore, a transfer system in which toner is transferred onto and off of a seamed intermediate transfer belt must be designed to operate over a wide range of electrical properties.
One method of compensating for the wide variations of the electrical properties of intermediate transfer belts is to use a "set point control" approach. For example, a transfer setpoint, such as an applied voltage or field-generating device, can be adjusted to compensate for environmental effects such as temperature and relative humidity that would otherwise change the intermediate transfer belt's electrical properties. Such an approach is effective because the electrical property changes due to the environment are substantially the same at all points along the belt. In general, the "set point" control approach enables a wider tolerance in the electrical properties of the intermediate transfer belt, provided those properties do not greatly vary along the belt's periphery. However, the set point control approach loses effectiveness when the electrical properties of the intermediate transfer belt vary over small distances, such as across a seam gap. Therefore, a seamed intermediate transfer belt suitable for receiving and transferring toner onto and off of its seam would generally require seam electrical properties that maintain a close relationship to the changing electrical properties of the rest of the belt. This presents a problem because the electrical properties of many otherwise good seam adhesives may not have the same responses as the rest of the belt.
While the difficulties of transferring toner onto and off of an intermediate transfer belts are numerous, so are the advantages. Therefore, an electrophotographic marking machine having an imageable seam intermediate transfer belt would be beneficial.